Various techniques have been investigated and utilized for attaching conductive material to semiconductor chips. These methods include the lift-off process, thru-mask methods, metal reactive ion etch (RIE) and metal and insulator damascene and various combinations of the above-listed methods. The lift-off and thru-mask methods are more valuable for large features, such as those typically encountered in chip packaging. Unlike the lift-off process and the thru-mask methods, the metal RIE and insulator damascene methods have been the process of choice for chip metallizations where the ground rules are typically below one micron.
In accordance with the damascene process, metal film may be deposited over the entire patterned substrate surfaces to fill trenches and vias. This metal deposition may then be followed by metal planarization to remove metal overburden and to isolate and define the wiring pattern. When metal deposition is by electroplating or by electroless plating processes, the plating may be preceded by the deposition of a plating base or seed layer over the entire surface of the patterned wafer or substrate. Also, layers that may improve adhesion, and/or prevent conductor/insulator interactions and/or interdiffusion may be deposited between the plating base or seed layer and the insulator.
In the metal RIE methods, blanket metal film is etched to define the conductor pattern. The gaps between the metal lines and vias are then filled with insulators. In high performance applications, the dielectric is planarized to define a flat metal level. One of the main advantages of the damascene process as compared to metal RIE is that it is often easier to etch an insulator as opposed to metal. Also, insulator gap fill and planarization may be more problematic.
In the metal damascene process, all the recesses in the insulator are first filled with metal before metal polishing. However, during the metal deposition into trenches and vias, all the narrower features become filled before their wider counterparts. Thus, all features with width less than 2 microns will be filled before those with width greater than 5 microns. Hence, to fill trenches or test pads with width of 50 microns, the smaller recesses typically with widths less than 5 microns are overplated. During metal chemical-mechanical polishing (CMP), the additional time needed to remove the excess metal overburden on the overplated smaller features causes dishing on the larger features. Also, because of the prolonged polishing times, insulator adjacent may severely erode. Severe dishing and insulator erosion in large metal features is a source of yield loss, especially when the occur at lower levels. Here they cause trapped metal defects at the next higher level. The longer time needed to remove the thicker metal overburden of the smallest metal lines and vias is one of the main culprits responsible for the low thruput and yield losses in the metal CMP process.
Moreover, this last metal wiring level, which may include Cu, Al, Au, Ni, W, α-Ta, and/or other metals and/or alloys with low resistivity, typically contains very wide metal lines for power bussing and large pads for wirebonds or C4 solder balls. In the CMP process, these relatively large metal structures are sensitive to dishing because of the prolonged polishing time.
After formation of wiring according to the above-described processes, further processing may be carried out on the substrate. For example, conductive elements, such as solder balls may be attached to portions of the wiring. One example of conductive elements that may be attached include solder balls for a C4 process.
Typically, patterns for a C4 process are created utilizing standard lithography techniques. According to such processes, a seed layer may be patterned, followed by lead-tin plating.